The present invention relates generally to an apparatus and method for improved sealing within bipolar plates used in a fuel cell assembly, and more particularly to the use of improved metal bead seals with compact designs and non-symmetric stacking arrangements.
Fuel cells convert a fuel into usable electricity via electrochemical reaction. A significant benefit to such an energy-producing means is that it is achieved without reliance upon combustion as an intermediate step. As such, fuel cells have several environmental advantages over internal combustion engines (ICEs) for propulsion and related motive applications. In a typical fuel cell—such as a proton exchange membrane or polymer electrolyte membrane (in either event, PEM) fuel cell—a pair of catalyzed electrodes are separated by an ion-transmissive medium (such as Nafion™) in what is commonly referred to as a membrane electrode assembly (MEA). The electrochemical reaction occurs when a first reactant in the form of a gaseous reducing agent (such as hydrogen, H2) is introduced to and ionized at the anode and then made to pass through the ion-transmissive medium such that it combines with a second reactant in the form of a gaseous oxidizing agent (such as oxygen, O2) that has been introduced through the other electrode (the cathode); this combination of reactants form water as a byproduct. The electrons that were liberated in the ionization of the first reactant proceed in the form of direct current (DC) to the cathode via external circuit that typically includes a load (such as an electric motor, as well as various pumps, valves, compressors or other fluid delivery components) where useful work may be performed. The power generation produced by this flow of DC electricity can be increased by combining numerous such cells into a larger current-producing assembly. In one such construction, the fuel cells are connected along a common stacking dimension—much like a deck of cards—to form a fuel cell stack.
In such a stack, adjacent MEAs are separated from one another by a series of reactant flow channels, typically in the form of a gas impermeable bipolar plate that—in addition to promoting the conveyance of reactants, coolant and byproducts—provides structural support for the MEA, as well as electrical current collection or conveyance and cell-to-cell sealing. In one common form, a typical automotive fuel cell stack may employ 100 or more bipolar plates, where the channels defined therein are of a generally serpentine layout that covers the majority of the opposing generally planar surfaces of each plate. The juxtaposition of the plate and MEA promotes the conveyance of one of the reactants to or from the fuel cell, while additional channels (that are fluidly decoupled from the reactant channels) may also be used for coolant delivery. In one configuration, the bipolar plate is itself an assembly formed by securing a pair of thin metal sheets (called half-plates, or more simply, plates) that have the channels stamped or otherwise integrally formed on their surfaces to promote fluid engagement. The various reactant and coolant flowpaths formed by the channels on each side typically convene at a manifold (also referred to herein as a manifold region or manifold area) defined on one or more opposing edges of the plate. Examples of all of these features—as well as a typical construction of such bipolar plate assemblies that may be used in PEM fuel cells—are shown and described in commonly-owned U.S. Pat. Nos. 5,776,624, 7,186,476 and 8,679,697, the contents of which are hereby incorporated by reference.
In a typical bipolar plate construction, a single plate assembly is formed from individual stamped plate layers that are welded or otherwise secured together, resulting in a laminated structure that defines the various fluid passages, support structures and electrically conductive surfaces. Historically, the sealing function around the plate manifolds, active area and fluid passages is achieved in one way through the use of separate gasket or seal assemblies where the underlying plate acts as a carrier, while in another way through cure-in-place (CIP) sealing materials placed between the adjacent plate layers during the assembly process.
Unfortunately, commercial automotive fuel cell applications require high volume manufacturing solutions that can produce 10,000 to 100,000 fuel cell stacks per year. Given that each cell requires a bipolar plate assembly on both opposing surfaces of the MEA, even low volume production would require more than a million plates be made. As such, both the CIP-based and gasket-based sealing approaches would be a cost-prohibitive way to achieve the sealing methods needed to reduce reactant or coolant channel flow losses, and as such are not suitable for high volume bipolar plate production.
To overcome some of the cost and manufacturing issues related to these sealing approaches, the Assignee of the present invention has developed integrally-formed bipolar plate sealing where the plate surfaces are stamped to produce outward-projecting metal bead seals (MBS) to establish discreet contact points between adjacent plate surfaces. While such a configuration is more compatible with the high-volume production needs mentioned above, their generally mirror-image (i.e., symmetrical) placement about a common plane that lies orthogonal to a plate stacking axis, coupled with their relatively large MBS width and the inherent manufacturing and assembly tolerances, has made them particularly susceptible to misalignment a crowning problems during fuel cell stack formation. In particular, under the compressive forces used in the stack assembly process, the flat MBS tops lead to the formation of concave-shaped regions within the adjacent MBS sealing surfaces rather than the intended (and generally uniform) face-to-face deformation. This in turn causes the contact pressure to be highly unevenly distributed on the seal surface, with high pressure at the edge and low pressure in the middle, resulting in reducing the effective seal surface contact area and lowering the ability of the MBS to perform its intended sealing function. To correct for the ensuing tendency to leak, other non-ferrous seal methods have to be used, such as through the use of generally complaint seals that significantly add to the cost of the assembled stack in a manner generally similar to the CIP approach discussed above.